librustc: Forbid .. in range patterns.

This breaks code that looks like:

    match foo {
        1..3 => { ... }
    }

Instead, write:

    match foo {
        1...3 => { ... }
    }

Closes rust-lang#17295.

[breaking-change]

Author: pcwalton

reference.md

@@ -3410,7 +3410,7 @@ may be specified with `..`. For example:
 
 let message = match x {
   0 | 1  => "not many",
-  2 .. 9 => "a few",
+  2 ... 9 => "a few",
   _      => "lots"
 };
 ```